Plague Killed People 3,300 Years Earlier Than Thought

The photo shows a Bronze Age human skull from the Yamnaya culture painted with red ochre. Yamnaya later developed into the Afanasievo culture of Central Asia, one of the cultures that carried the early strains of Y. pestis. Credit: Rasmussen et al./Cell 2015

New Research shows that the plague infection were already common much earlier than previously thought.

Here is a Halloween costume idea. Go as plague infected human from the Bronze Age. A new study published today found that the plague has infected humans much earlier than previously believed.

Through DNA analysis of tooth samples from Bronze Age human remains from Europe and Asia, the researchers discovered evidence of plague infections roughly 4,800 years ago. But it was at least another thousand years until the bacterium that causes the disease, Yersinia pestis, acquired key changes in virulence genes, allowing it to spread via fleas and evade the host immune system.

"We found that the Y. pestis lineage originated and was widespread much earlier than previously thought, and we narrowed the time window as to when it developed," says senior study author Eske Willerslev of the Center for GeoGenetics, University of Copenhagen. "This study changes our view of when and how plague influenced human populations and opens new avenues for studying the evolution of diseases."

Y. pestis was behind the sixth century's Plague of Justinian, the Black Death, which killed 30%-50% of the European population in the mid-1300s, and the Third Pandemic, which emerged in China in the 1850s.

Willerslev, Kristian Kristiansen of the University of Gothenburg, and their collaborators suspected that the plague could have shaped human populations much earlier than previously thought.

A few months ago, they published a high-profile population genomics study of Eurasian individuals from the Bronze Age (c. 3000 BC to 1500 BC), which they showed was a highly dynamic period involving large-scale migrations and population replacements that were responsible for shaping major parts of present-day demographic structure in both Europe and Asia.

But the reason for these migrations was not clear. "One of the scenarios we discussed was the idea that large epidemics could have facilitated such dynamics," says study co-first author Morten Allentoft of the Center for GeoGenetics, University of Copenhagen. "Perhaps people were migrating to get away from epidemics or re-colonizing new areas where epidemics had decimated the local populations. Could it be, for example, that plague was present in humans already in these prehistoric times?"

The researchers screened 89 billion raw DNA sequence reads obtained from the teeth of 101 Bronze Age individuals from Europe and Asia to find an answer. These teeth were obtained from various museums and archaeological excavations.

They discovered Y. pestis DNA in seven of these individuals, whose teeth were dated between 2794 BC and 951 BC (early Iron Age). Evolutionary analysis revealed that the most recent common ancestor of all known Y. pestis strains is 5,783 years old--thousands of years older than previous estimates.

In future studies, the researchers will look for evidence of plague in other geographic regions and time periods to get a better grasp of the history of this disease. They will also search for ancient DNA remains of other blood-borne bacteria and viruses.

"Our findings reveal that one can find ancient pathogenic microbes in ancient human material showing no obvious morphological signs of disease," Willerslev says. "So plague is just one disease to look at, and one could explore all kinds of diseases like this in the future."

The paper titled "Early Divergent Strains of Yersinia pestis in Eurasia 5,000 Years Ago" was published in Cell.

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